1. Field
Embodiments of the invention relate to the field of piezoelectric resonators; and more specifically, to the suppression of passband ripple in bulk acoustic wave resonators.
2. Background
Piezoelectric resonators are primarily used for signal filtering and reference oscillators. These resonators are commonly referred to as FBAR (film bulk acoustic resonators) or BAW (bulk acoustic wave resonators).
The resonator consists of piezoelectric material (i.e., aluminum nitride, AlN) sandwiched between two electrodes as shown in FIG. 1. For good performance the resonator must be acoustically isolated from the mechanical substrate (typically a silicon wafer). This is accomplished by an air gap (FBAR) or a Bragg mirror for solidly mounted resonators (SMR) of alternating high and low acoustic impedance materials designed to be one-quarter wavelength thick (λL/4) at the operating frequency. These devices are not new and are well documented in the literature. (See “Face-mounted piezoelectric resonators”, W. E. Newell, Proc. IEEE, Vol. 53, June 1965, Pgs. 575-581, U.S. Pat. No. 5,373,268, “Stacked Crystal Filters Implemented with Thin Films”, K. M. Lakin et al., 43rd Ann. Freq. Contr. Symp., May 1989, Pgs. 536-543 and “Advancement of MEMS into RF-Filter Applications”, R. Aigner et al., Proc. of IEDM 2002, San Francisco, Dec. 8-11, 2002, Pgs. 897-900.) The following is more specific to the SMR class of BAW devices.
Bragg mirrors have been developed in both microwave and optical applications to create a high reflection coefficient at a specified interface. In the case of the BAW resonator, the interface of interest is between the bottom electrode of the BAW resonator and the top of the Bragg mirror stack. An ideal Bragg mirror stack would create a unity reflection coefficient at this interface and perfectly isolate the BAW resonator from the substrate. In that case the substrate (typically silicon) would have no influence on the performance of the BAW resonator. This is not achievable in practice, since for an acceptable level of isolation (i.e., nominally 99% or better reflection coefficient) at the interface, several layers of high and low impedance materials (also called bi-layers) are required, which is generally not practical from a processing or cost standpoint. Typically only 2 to 3 bi-layers are practical, which allows some leakage of acoustic energy into the substrate that manifests itself as loss in the passband. Of interest to this invention is that when the substrate is an odd multiple of quarter wavelengths thick, it can present a boundary condition at the bottom of the Bragg mirror stack that will cause the required reflection condition to fail, causing ripples in the passband. These ripples are harmonically related to the substrate thickness T. Both of these phenomena are shown in FIG. 2. This condition can be prevented if a lossy material having an optimal acoustic impedance is applied to the substrate backside.
Referring to FIG. 2, the acoustic energy that leaks through the Bragg mirror causes two problems. The first is increased insertion loss due to energy leakage out of the BAW resonator, as illustrated by arrow 1. The second is reflections of acoustic energy off of the substrate backside, causing a boundary condition at the Bragg mirror backside that results in ripples in the BAW resonator passband, at frequencies where the substrate is an odd number of quarter wavelengths thick, as illustrated by arrow 2.
It has been proposed that BAW resonator performance can be improved by roughening the substrate backside, by either mechanical or chemical means (See “Face-mounted piezoelectric resonators”, W. E. Newell, Proc. IEEE, Vol. 53, June 1965, Pgs. 575-581 and U.S. Pat. No. 5,373,268) and/or adding an epoxy (See “Face-mounted piezoelectric resonators”, W. E. Newell, Proc. IEEE, Vol. 53, June 1965, Pgs. 575-581) or other material to that rough backside as shown in FIG. 3. The theory is that the acoustic energy that leaks through the Bragg stack will be either “partially scattered” by the rough surface or absorbed by the epoxy (or other absorptive material). This will in turn improve the passband performance by reducing the amplitude of acoustic energy reflected back into the bottom of the Bragg stack and hence reduce passband ripple.
FIG. 3 schematically illustrates a substrate with a rough backside and absorber on the backside. Acoustic energy that leaks through the Bragg mirror stack is scattered by the rough surface (1) and absorbed (2). The benefit is reduced ripple due to attenuation of reflected acoustic energy into the BAW resonator (3). The penalty is energy loss in the passband due to scattered and absorbed energy.